Jo Ann Sacrcino, age 11, of Thorold, Ontario, Canada, for her question:

Why do birds cast their feathers?

The most sensible clothing of the entire animal kingdom is worn by the birds. What's more, some of them wear the most gorgeous attire in the world. However, birds spend a lot of time preening their plumage and at least once a year all their feathers must be replaced with new ones.

Discarded feathers are strewn around most often in the fall, after the nesting season, when many birds are preparing to fly south to warmer winter climates.. We also find lots of their lost feathers in springtime, at the beginning of the nesting season. Almost always, the birds cast these feathers because new ones are growing in to replace them. A lost feather may appear, at least to our eyes, to be in good condition. You may think that it is sheer extravagance for the bird to cast away perfectly good clothing in this way. But this is not true at all. The voice of nature does not in¬struct her children to waste the good things provided for them. After months of heavy duty, each fragile feather is either worn or unsuitable for the months ahead.

Almost every discarded feather is a lovely little object to behold. But its true wonders are too small for our eyes to see and we need to view it under a microscope. Suppose you find a quill discarded by a pigeon. It is shaped like a long, stiff leaf and is light as a feather    naturally. Its leaf like vein is a hollow, horny shaft bordered on both sides with fringes of soft hairs. Those hairs are springy barbs that tend to cling together like bands of silken ribbon.

This is because each barb is bordered with smaller barbules and each barbule has smaller barbicels and hooks. Our pigeon quill may have a hundred thousand barbules with many millions of barbicels and hooklets. All these microscopic fragments of the feathery finery can be hooked together and unhooked for cleaning and preening purposes. Feathers are warm and light and surprisingly durable. But imagine their heavy duties, the wear and tear of flying and other activities through wind and weather. The dainty threads are preened daily and usually smoothed with oily moisture, but broken and bruised pieces cannot be repaired. The average feather lasts a few months and some last a year. Then the bird discards it and a new one grows in to replace it.

The main reason why birds cast their feathers is because they are worn out. In spring, they may have another reason. Birds that stay around all winter need extra warm clothing. A sparrow may have 3,500 feathers in his winter plumage. This coat is far too warm for the summer so when spring comes he may cast away his excess winter feathers. Many male birds like to doll up in bright new outfits in the spring. When the courting season begins, they shed their shabby winter feathers and fresh ones replace them.

Every bird molts all his feathers at least once every year. As a rule the molting is done in stages over several weeks. Flying birds molt their important tail and wing feathers in pairs, one from the right and one from the left side. Other feathers fall out while newer ones are partly grown. A chicken wears about 8,000 feathers, a humming¬bird has at least 1,000 and a swan has about 25,000. Songbirds wear from 1,100 to 4,600. With all these feathers being discarded each year, it is little wonder we find so many of them strewn around on the ground.